1. Field of the Invention
The present invention relates to a mobile handset capable of determining its current position by using radio waves.
2. Description of the Background
Various methods by which a mobile handset may determine its current position, for example, using the Global Positioning System (GPS), have been used in practical applications, such as car navigation. A method also exists in which, in conjunction with the method using GPS, a cellular telephone network is used to notify a mobile handset of auxiliary information for receiving radio waves for GPS. See, Nikkei Communications, issued by Nikkei Business Publications, Inc., Jul., 16, 2000 at p. 120.
An additional method is known in which a mobile handset receives radio-frequency (RF) carriers transmitted from base stations for cellular telephony communications, and the handset calculates its current location using the propagation delay time of the RF carriers. See, Japanese Patent Application No. JP-A-181242/1995.
As radio signals from satellites for GPS propagate and reach the surface of the earth, the intensity of the signals decreases. Particularly indoors, the signal intensity greatly attenuates because of blockage by ceilings, walls, and the like, such that the received signal quality is significantly deteriorated. Because of this, it is difficult for a mobile handset using the above GPS method to receive sufficient quality GPS signals required for determining the position of the handset in indoor environments. In other words, the mobile handset cannot accurately determine its position if indoors.
Additionally, when the handset is in proximity to a tall building or other obstruction, a direct view of the sky may be limited. Consequently, the number of GPS satellites which meet the received signal quality requirements for practical position determination is low in theses areas, and it is difficult for a mobile handset to determine its position by the GPS method.
At the same time, the position determination method using RF carriers from base stations for cellular telephony communications has the following limitation. If a mobile handset is used very near a base station or in a rural or mountainous area where a limited number of base stations exist, the number of base stations which transmit signals that meet received signal quality requirements for practical position determination is low at the mobile handset's location. Consequently, the above method can not be used in such circumstances.
When a mobile handset attempts to determine its position by either the GPS method or the method using RF carriers from cellular base stations, there may be some locations where an accurate position determination can not be established. Consequently, location information serviceable areas may be restricted by conventional methods.